1. Technical Field
The present invention is related to drill bits for boring earthen formations. The present invention is particularly adapted for rolling cutter earth-boring bits most typically used in oil and gas drilling, but also has application in bits used in blast hole and mining applications.
2. Summary of the Prior Art
In 1909, Howard R. Hughes invented the rolling cutter rock bit, which revolutionized the exploration and drilling of oil and gas wells. Since that time, countless improvements have been made to Hughes' basic design.
One problem that remains to be solved is that of “tracking.” Tracking occurs when a cutting element (tungsten-carbide insert or steel tooth) falls in the same impression that was made previously by the same or another cutting element. This results in loss of drilling efficiency since the primary mode of contact between cutters and formation is between the surface of the cutter and formation rather than between the cutting elements and formation. This results in increased wear of the bit as well as reduction in feet per hour or penetration rate.
Conventional solutions to tracking include increasing the weight-on-bit (WOB), but, as can be expected, this reduces bit life because of the additional strain on bit components. Probably the most common way to reduce tracking and vibration is to decrease the pitch between adjacent cutting elements or increase cutting element count, especially for hard rock formations as shown in U.S. Pat. Nos. 6,161,634 and 3,726,350. The disadvantage of such solutions is that overbreak effect is not utilized, specific energy increases and the cost of the drill bit is augmented.
Tracking also can be partially reduced by increasing sliding and scraping of cutting elements on the bottom hole by adjusting the geometry of the bit. The drawback of this approach is that the cutting elements that are sliding and scraping will wear faster while tracking will not be completely eliminated.
Another solution to the tracking problem is the use of varying pitch (angular distance between the centerlines) between the cutting elements for instance as proposed in U.S. Pat. Nos. 4,248,314, 4,187,922 and 3,726,350. Any deviation from equal pitch, can dramatically increase bit vibration, again causing premature bit wear. Moreover, merely randomly varied pitch drill bits can track just as much as equally spaced drill bits.
Tracking can also be reduced through various configurations of cutting elements or teeth, including teeth with “T” shape crest for additional wear resistance wherein the teeth/inserts crush the formation to reduce tracking (for example see UK Patent number 3,326,307). This approach tends to reduce drilling speed and increases specific energy (energy applied per unit of formation broken) because the cutting elements crush the formation with lower penetration rate. Another variation is to group and space cutting elements with varied pitches between groups in combination with changing the orientation of the cutting element crests for various groups. (See for example UK Patent 1,896,251). These approaches may reduce tracking; however they increase manufacturing cost. See U.S. Pat. No. 2,333,746. A change in cutting element orientation as shown in U.S. Pat. No. 4,393,948 without optimal placement on the surface of the cutters can only reduce but not completely eliminate tracking.
Methods to optimize drill bit performance using simulations and other statistical data to improve performance parameters of the bit are illustrated in U.S. Pat. Nos. 6,213,225; 6,095,262; 6,516,293; and published patent applications 20,030,051,917; 20,030,051,918; 20,010,037,902. Ad hoc simulation approaches are best implemented in the absence of adequate theory; however, statistical optimization results are limited by the assumptions and biases taken at the beginning of the optimization process. Furthermore, prior-art simulation methods have over-inflated the cutting element count required to optimally drill earthen formations.
A need exists, therefore, for an earth-boring bit having anti-tracking characteristics that avoids excessive vibration and can be economically produced.
One common drawback of all the prior art solutions is lack of overbreak optimization during drilling of rock formation. The overbreak effect is the investigation of the fact that rock has strong compression properties and has weak bending and distention properties as compared to metal, for instance iron.
Another common drawback all the prior art solutions is misunderstanding by those knowledgeable in the art of actual cause for detrimental axial resonance frequency vibration of the cutter drill bit by boring rock. Inventors found the actual cause for detrimental axial resonance frequency vibration for roller cutter drill bits for the first time since long history of improvements made to Hughes' basic roller cutter drill bits; found cause is eliminated in the present invention.